Acute viral hepatitis is a disease which may result in chronic liver damage. It is clinically diagnosed by a well-defined set of patient symptoms, including jaundice, hepatic tenderness, and an increase in the serum levels of alanine aminotransferase and aspartate aminotransferase. Serologic immunoassays are generally performed to diagnose the specific type of viral causative agent. Historically, patients presenting with symptoms of hepatitis and not otherwise infected by hepatitis A, hepatitis B, Epstein-Barr or cytomegalovirus were clinically diagnosed as having non-A, non-B hepatitis (NANBH) by default.
For many years, the agent of non-A, non-B hepatitis remained elusive. It has now been established that many cases of NANBH are caused by a distinct virus termed hepatitis C virus (HCV). European Patent Application EP-A-0318216 discloses CDNA sequences derived from HCV, polynucleotide probes and polypeptides for use in immunoassays. Further information is provided in European Application EP-A-0388232.
The HCV genome encodes a large polyprotein precursor, which contains structural and non-structural regions. The single protein is apparently cleaved into a variety of proteins after production. Most of the structural and non-structural proteins have now been identified from in vitro RNA translation and expression as recombinant proteins. The C and E regions encode for nucleocapsid structural proteins and for envelope structural proteins, respectively. At least five additional regions follow, which encode for non-structural (NS) protein of undefined function. The organization is believed to be as follows (A. Alberti, Journal of Hepatology, 1991; 12; 279 to 282)
5′                                          3′NCR: C : E1 : E2 : NS1 : NS2 : NS3 : NS4 : NS5Certain immunoreactive proteins have been described as recombinant proteins, for example C22 (in the core region), C33 (in NS3 region), 5-1-1 and C100 (both in the NS4 region), and NS5 (NS5 region). Diagnosis of hepatitis C is still largely based on methods which detect antibodies against the product of the C-100 clone. This clone was ligated with overlapping clones to produce a larger viral antigen (C100) corresponding to part of the NS3–NS4 genomic region. C100 was then fused with the human superoxide dismutase (SOD) gene, expressed in use as a large recombinant fusion protein (C100-3) and used on solid phase to develop radio-labelled (RIA) and enzyme-linked immunosorbent assays (ELISA).
Polynucleotides useful for screening for HCV are disclosed in European Patent Specification EP-A-0398748. European Patent Specification EP-A-0414475 purports to disclose the propagation of HCV in culture cells and the production of antigens for use in diagnostics. European Patent Specification EP-A-0445423 discloses an improved immunoassay for detecting HCV antibodies.
Blood banks in the United Kingdom have recently begun routine testing of blood donors for antibodies to components of HCV. One assay involves the detection of HCV antibodies to C100-3 polypeptides. The C100-3 antibody recognizes a composite polyprotein antigen within non-structural regions of the virus and is a consistent marker of HCV infection. However, in acute infections this antibody is unreliable because of the delay (typically 22 weeks) in seroconversion after exposure. Furthermore, the C100-3 antibody test lacks specificity for the hepatitis C virus.
Second generation antibody tests employ recombinant antigens or synthetic linear peptides representing structural antigens from the highly conserved core region of the virus as well as non-structural antigens. However, it is found that some second-generation ELISA tests can yield false-positive reactions. The recombinant immunoblot assay (RIBA-2) incorporating four antigens from the HCV genome, provides a method for identifying genuine-anti-HCV reactivity. However, the result can be “indeterminate.” The present workers have reported (The Lancet, 338; Oct. 19, 1991) varying reactivity of HCV-positive blood donors to 5-1-1, C100, C33C and C22 antigens, and compared these with the results of the direct detection of HCV RNA present in the blood samples using polymerase chain reaction (PCR) to amplify HCV polynucleotides. However, the work demonstrates that the unambiguous diagnosis of HCV infections is not yet possible.
Recently there has been discovered a second type of HCV (References 1, 2) called K2 that differs considerably in sequence from the published prototype (Reference 3) or the first type K1 sequences (References 4 and 5).